The basis of the Red Bull junior team’s car for next year was designed around the Renault power unit, but that deal is moving to McLaren for 2018.
Toro Rosso technical boss James Key said the Honda unit is “fundamentally different”.
“It’s a very compact unit, but a different architecture to the Renault. It requires a lot of adaptation work,” he told Speed Week.
Key said Toro Rosso cannot start the Honda-powered 2018 car “from scratch” because the main aerodynamic surfaces of the car are already set in stone.